The invention relates to a spring element, in particular a spring rail for wipers, as are used for the generally curved windshields of motor vehicles, rail-borne vehicles, ships and aircraft.
Wipers usually comprise a wiper lever with a wiper blade composed of a spring rail and a wiper strip which is pressed onto the screen to be cleaned with the aid of spring forces. To achieve the necessary cleaning action, it is necessary for elastomeric wiper strip always to bear tightly against the screen surface irrespective of the curvature of the screen. This is ensured by spring elements arranged between the wiper lever and the wiper strip, in particular including the spring rail, the length of which substantially corresponds to that of the elastomeric wiper strip.
However, high driving speeds and/or wind speeds give rise to turbulence and vibrations, with the result that the wiper strip does not bear uniformly and with sufficient force against the screen over its entire length and/or throughout its entire reciprocating motion, with the result that films of water and dirt adhering to the screen are not reliably removed.
Modern wipers comprise a main bracket which is arranged in articulated fashion on a motor-driven wiper lever and has a claw bracket articulatedly secured to each of its two ends. The claw brackets are articulatedly connected at one end to a spring rail and at the other end to claws, the two ends of which are in each case connected, via joints, to the spring rail. The spring rail is embedded in the elastomeric wiper strip over its entire length.
The spring system, which in total comprises five brackets and a spring rail, is intended to ensure that the wiper strip bears uniformly against the screen. To achieve this, and in particular to suppress rattling vibrations, the distances between the two claw brackets and the length of the latter have to be matched to the screen geometry. Further criteria are the size of the screen surface to be covered, the length of the wiper blades, the orientation of the reciprocating-motion axis of the wiper arm with respect to the screen surface and in particular the spring force and the width and thickness of the spring rail. It is scarcely possible to record these parameters by calculation; consequently, the nature of the claw brackets and their position with respect to the wiper blade are generally based on practical experience.
Despite all efforts, it has only to a certain extent been possible to avoid rattling and the occurrence of vibrations at high driving speeds and/or wind speeds. Accordingly, the result of wiping is unsatisfactory and, moreover, there is extensive abrasion to the wiping edge of the wiper strip, as well as disruptive operating noise, and furthermore the service life of the wiper strip is shortened.
To reduce the noise, European laid-open specification 1 288 089 A2 proposes reducing the coefficient of friction of a wiper strip with a special profile with the aid of a polymer coating. However, this is not only highly complex but also makes it easier for vibrations to occur as a result of the lower coefficient of friction. Furthermore, PCT laid-open specification WO 01/58732 A1 proposes using two spring rails running parallel to one another but with different resonant frequencies instead of a single spring rail extending over virtually the entire length of the wiper strip, in order to suppress the occurrence of wiper blade vibrations. However, different resonant frequencies require spring rails which differ in terms of their cross section and/or material, and therefore entail additional outlay both on production and on stock-holding and in terms of spare parts. An additional factor is that it is only possible to avoid vibrations by using two spring rails with different resonant frequencies within a relatively narrow frequency window, and therefore it is not possible to cover all operating or vibration states which occur in practise.
The material used for spring elements and spring rails is usually alloyed steels, since pure carbon steels have poor damping properties and therefore do not break down disruptive vibrations sufficiently quickly. This is because scarcely any energy-consuming processes take place within the microstructure.